Drilling well bores in the earth, such as well bores for oil and gas wells, is an expensive undertaking. One type of drilling system used is rotary drilling, which consists of a rotary-type rig that uses a sharp drill bit at the end of a drill string to drill deep into the earth. At the earth's surface, a rotary drilling rig often includes a complex system of cables, engines, support mechanisms, tanks, lubricating devices, and pulleys to control the position and rotation of the bit below the surface.
Underneath the surface, the drill bit is attached to a long drill string that transports drilling fluid to the drill bit. The drilling fluid lubricates and cools the drill bit and also functions to remove cuttings and debris from the well bore as it is being drilled.
Directional drilling involves drilling in a direction that in not necessarily precisely vertical to access reserves that are not directly beneath the drilling rig. Directional drilling involves turning of the drill bit while within the well bore. Off shore drilling often involves directional drilling because of the limited space beneath the offshore platform, although direction drilling is also vastly used on shore.
Various types of directional drilling tools exist. After a portion of a well is drilled, the drill bit is turned off, and a whip stock is inserted into the well bore to push the drill bit in the desired direction. This procedure is time consuming because the drill bit cannot rotate when it is being steered.
Another type of direction drilling involves bent subs in which a slight curvature of a bent sub steering of the drill string. To steer, rotation of the drill string is halted, but the drill bit continues to rotate powered by an associated mud motor. Because the bent sub is slightly angled and because the drill string is not rotating, the drill string is effectively steered in the direction of the bend of the bent sub. A measurement while drilling (MWD) system may be used such that accurate measurements may be made of the direction and location of the drill string.
Another type of direction drilling involves rotary steerable directional drilling, in which the drill string continues to rotate while steering takes place. Typically, a plurality of steering ribs are associated with the rotary steerable directional drilling tool to facilitate the steering. The ribs are disposed outwardly from a sleeve, inside of which is disposed a rotating shaft associated with the drill string. In one type of rotary steerable directional drilling tool, the outer sleeve rotates and in another the outer sleeve does not rotate. In the type in which the outer sleeve does not rotate, bearings allow relative movement between the outer sleeve and the rotating shaft.